Chris Porter struck twice in the dying seconds to down Nottingham Forest and set up a possible Steel City FA Cup derby
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Harry Maguire's first-half brace helped give new Sheffield United manager Nigel Clough a dream start.
The 20-year-old defender twice headed home unmarked in the opening half-hour at Bramall Lane before Ryan Flynn sealed the three points.
Alex substitute Max Clayton grabbed a late consolation as his side slipped into the relegation zone, below the Blades on goal difference.
Sheffield United are expected to confirm the appointment of Nigel Clough as their new manager at a press conference at Bramall Lane on Wednesday afternoon
The Blades sacked David Weir 12 days ago following a run of just one win in 13 matches and Clough is poised to become the club's eighth permanent manager in the last six-and-a-half years.
Clough, 47 is understood to have agreed a severance package with Derby, who sacked him at the end of last month after nearly four years at Pride Park.
I can see that I've been linked with the vacant managers job at Sheffield United. (Cont)
..this is not my doing, I have not applied for any jobs, but it's great to be associated with such a great football club.
Sheffield United have completed the signing of former Birmingham striker Marlon King.
The 33-year-old, who left Birmingham by mutual consent in August, has signed a deal that will keep him at Bramall Lane until the end of the season.
Blades manager David Weir told the club's official website: "Marlon is vastly-experienced and has scored goals throughout his career, many of which that have come in the Championship"